In a recent large scale liquid crystal display, a direct-lit type backlight has been adopted, wherein a plurality of Cold Cathode Fluorescent Lamp (CCFLs) or LEDs (Light Emitting Diode) are arranged. A diffusion plate having strong light scattering properties is used between an image display device and a light source so that CCFLs or LEDs can not be visually recognized. A diffusion plate diffuses light in all directions by a light diffusion effect darkening a liquid crystal display device. In addition, a plate thickness of a diffusion plate is usually required to be 1-5 mm in order that light scattering properties are improved and the diffusion plate supports an optical film arranged on the diffusion plate. Therefore, light is absorbed by a diffusion plate, and a liquid crystal screen display is darkened.
In addition, year by year, liquid crystal televisions are becoming thinner. Thereby, a diffusion plate also becomes thin and further improvement of diffusion property is required.
Conventionally, the purpose of a diffusion plate used for a direct-lit type backlight is to diffuse light from a CCFL of a light source and to reduce brightness non-uniformity (a lamp image). However, it is difficult to perfectly remove the lamp image.
In the case where diffusion particles are increased too much in order to perfectly remove the lamp image, Total Luminous Transmittance is significantly reduced causing a reduction in brightness. In addition, if diffusion particles of a diffusion plate are reduced in order not to reduce Total Luminous Transmittance, a diffusion effect is also reduced.
In patent documents 1-3, an example is disclosed as a means for improving diffusion performance. In the example, a lens shape is provided to a light outgoing surface of a diffusion plate. For example, a lens having a convex curved surface is arranged on a diffusion plate. In such a diffusion plate, it may be necessary that a shape of a lens is designed so as to correspond to an arrangement of a light source, and determine the alignment of the lens. Therefore, a manufacturing process may become complex. In addition, if a lens shape is provided to a light outgoing surface of a diffusion plate, Total Luminous transmittance of a diffusion plate may be reduced, thereby a liquid crystal display screen may be darkened. Further, a lens sheet arranged on a diffusion plate and a liquid crystal pixel may cause moire interference.
As means to improve brightness of a liquid crystal display screen, a brightness improving film (Brightness Enhancement Film: BEF, a registered mark of 3M (Amerika)) is broadly used as a lens sheet. FIG. 17 is a schematic cross sectional diagram showing one example of the arrangement of a BEF. FIG. 18 is a perspective view of a BEF. As shown in FIGS. 17 and 18, BEF 185 is an optical film in which a unit prism 187 having a triangle cross sectional shape is periodically arranged in one direction on a member 186. This unit prism 187 has a large size (pitch) compared with a wavelength of light.
BEF 185 collects light from “off-axis”, and can redirect or recycle this light “on-axis” toward a viewer. That is, BEF 185 can increase brightness on an axis by reducing brightness off an axis when a display is used (viewed). Here, “on axis” is a direction corresponding to a view field direction F of a viewer, and, in general, is the side of a direction of a normal line of a display screen.
However, in the case where BEF 185 is used, the light component due to reflection/refraction may simultaneously be emitted toward a lateral direction without progressing towards a viewing direction F of a viewer. Line B in FIG. 19 shows the characteristics of BEF 185. Light intensity in a direction on axis (the angle to a view field direction F′ is 0°) is increased the most. However, there is a small light intensity peak (side lobe) around 90° (the angle to a direction F′ is 90°). Light emitted in a lateral direction is increased without progressing to a viewer.
In the case where a lens sheet represented by a BEF 185 is used, a diffusion filler is applied to a transparent base material, a diffusion film (hereinafter, called “an under diffusion film”) having both functions of diffusion and light collection is arranged between a diffusion plate and a lens sheet, thereby diffusion light emitted from a diffusion plate is effectively collected while visibility of a light source which can not be perfectly removed just by a diffusion plate can be controlled.
Further, in the case where a light diffusion film is arranged between a lens sheet and a liquid crystal panel, a side lobe can be reduced while moire interference occurring between a regularly arranged lens and a liquid crystal pixel can be prevented. However, in a type using an under diffusion film and a light diffusion film, the number of components is increased, thereby processes in manufacturing a display become complex and dust becomes mixed between optical sheets.
In patent document 4, the following are disclosed as a means for solving such a problem. Without using an optical film comprised only of the unit prism, an optical film having an array structure in which a unit lens is arranged in a two dimensional direction with a constant pitch is used for a backlight unit. The optical characteristics of this optical film are shown by the line A in FIG. 19, a side lobe is not shown, and light intensity in a viewing field direction F′ is improved more than a line B.
However, in a backlight unit using such an optical film, because it is necessary that a light outgoing surface of a diffusion plate is flat in order to laminate an optical film to form one body, it was difficult to improve a diffusion effect and a light collecting effect by providing a light outgoing surface of a diffusion plate with a lens.    Patent document 1: JP-A-2007-103321    Patent document 2: JP-A-2007-12517    Patent document 3: JP-A-2006-195276    Patent document 4: JP-A-2007-213035